As a non-aqueous lithium ion secondary battery for general use, a "rocking chair type" battery (which comprises a combination of lithium ion retaining/releasing materials) is widely employed in these days. The battery generally comprises a negative electrode of one or more of various carbonaceous materials (negative electrode active material) to which lithium ions can be reversibly intercalated, and a positive electrode of lithium-containing metal complex oxide (positive electrode active material) which can reversibly capture and release lithium ions.
As the positive electrode active materials, LiCoO.sub.2, LiCo.sub.1-x Ni.sub.x O.sub.2, LiNiO.sub.2 and LiMn.sub.2 O.sub.4 have been generally employed. LiCoO.sub.2 is advantageously used because it gives a high voltage of more than 3.5 volts in combination of Li, and a high discharge capacity (shown in Japanese Patent Provisional Publication No. 55-136131). Japanese Patent Provisional Publications H3-147276 and H4-123769 disclose a secondary battery comprising a positive electrode active material of LiMn.sub.2 O.sub.4. Since manganese is less expensive and easily available than cobalt, the disclosed battery can be produced at a relatively low cost.
As the carbonaceous material for negative electrode active materials, graphite material, pitch coke, fibrous carbon, and soft carbon obtained by low temperature firing are known. However, since the carbonaceous material generally has a low bulk density of less than 2.20, the resultant battery shows a low discharge capacity even if the material is made to retain lithium ions at the stoichiometric maximum of intercalating capacity (372 mAh/g). In order to solve the problem, some publications propose negative electrode active materials of amorphous metal complex oxide mainly comprising tin oxide (Japanese Patent Provisional Publications No. H6-60867, No. H7-220721, No. H7-122274 and No. H7-288123, and PCT WO96-33519).
The above-mentioned amorphous metal complex oxide (negative electrode active material) in combination with cobalt oxide (positive electrode active material) gives a battery of high energy density, but thus composed battery is considerably expensive. On the other hand, a battery comprising a negative electrode of the amorphous metal complex oxide and a positive electrode of manganese oxides has good cost performance, but its energy density is considerably small.
Published European Patent Application No. 0651450 discloses a battery comprising a negative electrode of the amorphous metal complex oxide and a positive electrode of nickel oxide (such as LiNiO.sub.2 and Li.sub.x Co.sub.a Ni.sub.1-a O.sub.2). According to the above publication, the battery comprising the nickel oxide has both good cost performance and a high discharge capacity. However, the battery employing LiNiO.sub.2 (basic component of the positive electrode of nickel oxides) generally has such drawbacks that the average discharge voltage is lower than that of the battery using LiCoO.sub.2 by 0.2 volt or more, and that its cycle capacity retention is relatively poor. Because of the low discharge voltage, LiNiO.sub.2 cannot effectively work in the low voltage range so that the capacity of the battery is restricted to an unsatisfactory level.
Accordingly, it is an object of the present invention to provide a non-aqueous lithium ion secondary battery having both high discharge capacity and good cost performance.